1. Field of the Invention
The present invention relates to aliphatic polyurea elastomers.
2. Description of Background Art
Polyurea elastomer systems are commonly recognized as coating materials, with spray polyurea elastomer systems being particularly useful when employed in this capacity. One of the shortcomings associated with aromatic polyurea elastomer systems, which generally are prepared by reacting an aromatic isocyanate with an active hydrogen component in the presence of an aromatic chain extender, is that they exhibit poor stability when exposed to ultraviolet radiation. This becomes particularly problematic when the substrate to be coated, for example, a rooftop, is continuously subjected to ultraviolet exposure. The resulting ultraviolet degradation of the elastomer system is typically manifested by a change in color; a general loss of product integrity, such as cracking; and an adverse reduction in properties, such as tensile strength, tear strength and elongation, to name a few.
The most widely employed aromatic chain extender is diethylenetoluenediamine (DETDA), a product of Ethyl Corp. In addition to providing an ultraviolet unstable system, DETDA provides a rigid elastomer system which, because of the rigidity, has difficulty in assuming the detail or contour of the substrate to be coated. However, polyurea elastomer systems fabricated from, among other things, DETDA generally exhibit good processing characteristics. Thus, there is a recognized need for a polyurea elastomer system that exhibits ultraviolet stability and increased flexibility, i.e., less rigidity, while exhibiting good processing characteristics.
To provide an ultraviolet stable system, however, non-aromatic, i.e., aliphatic, components should be employed. For instance, Rowton, R. L., CYANOETHYLATED POLYOXYPROPYLENEPOLYAMINES: POLYMER FORMERS OF UNIQUE REACTIVITY, Journal of Elastomers and Plastics, Vol. 9, October 1977, describes cyanoethylated polyoxypropylenepolyamines as the hydrogen component in polyurea systems to provide light stable systems. Rowton is silent with respect to employing aliphatic chain extenders.
Some of the known aliphatic chain extenders include trans-1, 4-diaminocyclohexane and 1,6-diaminohexane. These and other known aliphatic chain extenders react very rapidly with isocyanate and, therefore, cannot be used in spray systems, inasmuch as polymerization occurs so rapidly that the polymer is virtually unsprayable. Therefore, while the aliphatic polyurea elastomers of the present invention can be used in reaction injection molding (RIM) systems, it is to be noted that the advantage provided by the present invention is more noteworthy in spray systems, relative to other elastomer systems where sprayability is not at issue, such as RIM systems.
Other low molecular weight, linear primary amine chain extenders exhibit a rapid reactivity that result in poor mixing and elastomer cure.
Additionally, elastomer systems prepared with known aliphatic chain extenders have notoriously exhibited processing characteristics inferior to those exhibited by systems fabricated from DETDA.
U.S. Pat. No. 3,666,788 is directed to cyanoalkylated polyoxyalkylene polyamines which are specifically described as being useful as curing agents in polyurea coatings. Similarly, U.S. Pat. No. 3,714,128 describes cyanoalkylated polyoxyalkylene polyamines which are useful for slowing the gelling or hardening of the polyurea component so that good mixing of the isocyanate and amine components can be attained, which gives the sprayed material ample time to adhere and level before gelation of the polyurea coating occurs. Neither of these patents describe the present cycloaliphatic diamines as chain extenders in aliphatic spray polyurea elastomer systems.
Polyoxyalkylene polyamines, particularly JEFFAMINE.RTM. T-403, D-400 and D-230, are described as chain extenders in U.S. Pat. No. 4,732,919; however, this patent relates to an aromatic system which is to be used in a RIM application.
Commonly assigned U.S. patent application Ser. No. 402,296, filed Sep. 5, 1989, describes aliphatic spray polyurea elastomers comprising an (A) component which includes an aliphatic isocyanate and a (B) component which includes (1) an amine terminated polyoxyalkylene polyol, and (2) an amine terminated chain extender.
Thus, it is my understanding that an aliphatic polyurea elastomer system which incorporates the particular aliphatic chain extenders described hereinbelow and which exhibits ultraviolet stability, increased flexibility and favorable processing characteristics has heretofore been unavailable.